Slice the cucumbers into thin rounds, about 1/8-inch thick. A mandoline works best for even slices. Place in a large bowl.
Halve the red onion lengthwise, then slice thinly crosswise into half-moons. Add to the bowl with cucumbers.
Add the chopped dill, parsley, and mint (if using) to the bowl.
In a small jar or bowl, combine the apple cider vinegar, olive oil, honey, salt, black pepper, and red pepper flakes. Shake or whisk until well blended.
Pour the dressing over the cucumber mixture and toss gently with a spatula or your hands until everything is evenly coated.
Let the salad rest at room temperature for at least 10 minutes before serving to allow flavors to meld. For best results, refrigerate for up to 2 hours. Serve cold or at room temperature.